In recent years, MIMO (Multiple Input Multiple Output) has been discussed as a transmission technique to improve frequency efficiency in radio communication. In MIMO transmission, both a transmitter and a receiver transmit and receive radio signals through space multiplexing using multiple antennas. In 3GPP (Third Generation Partnership Project), the MIMO transmission has been introduced in 3GPP TS 36.211 V8.9.0 (referred to as Release 8 hereinafter). Also, in 3GPP TS 36.211 V10.6.0 (referred to as Release 10 hereinafter), the MIMO transmission has been specified to, implement ZF (Zero Forcing), block diagonalization and so on.
In the MIMO transmission, a transmitter performs phase control (precoding) on each stream for transmission and transmits the phase controlled streams from multiple antennas. In this phase control, a precoding matrix selected based on feedback information (codebook) from a receiver is used. From this reason, the feedback information provided from the receiver is an important factor to implement the MIMO transmission.
For example, in Release 8, the receiver has 16 codebooks based on a table as set forth.
TABLE 1CodebookNumber of layers νindexun12340u0 = [1 −1 −1 −1]TW0{1}W0{14}/{square root over (2)}W0{124}/{square root over (3)}W0{1234}/21u1 = [1 −j 1 j]TW1{1}W1{12}/{square root over (2)}W1{123}/{square root over (3)}W1{1234}/22u2 = [1 1 −1 1]TW2{1}W2{12}/{square root over (2)}W2{123}/{square root over (3)}W2{3214}/23u3 = [1 j 1 −j]TW3{1}W3{12}/{square root over (2)}W3{123}/{square root over (3)}W3{3214}/24u4 = [1 (−1 − j)/{square root over (2)} −j (1 − j)/{square root over (2)}]TW4{1}W4{14}/{square root over (2)}W4{124}/{square root over (3)}W4{1234}/25u5 = [1 (1 − j)/{square root over (2)} j (−1 − j)/{square root over (2)}]TW5{1}W5{14}/{square root over (2)}W5{124}/{square root over (3)}W5{1234}/26u6 = [1 (1 + j)/{square root over (2)} −j (−1 + j)/{square root over (2)}]TW6{1}W6{13}/{square root over (2)}W6{134}/{square root over (3)}W6{1324}/27u7 = [1 (−1 + j)/{square root over (2)} j (1 + j)/{square root over (2)}]TW7{1}W7{13}/{square root over (2)}W7{134}/{square root over (3)}W7{1324}/28u8 = [1 −1 1 1]TW8{1}W8{12}/{square root over (2)}W8{124}/{square root over (3)}W8{1234}/29u9 = [1 −j −1 −j]TW9{1}W9{14}/{square root over (2)}W9{134}/{square root over (3)}W9{1234}/210u10 = [1 1 1 −1]TW10{1}W10{13}/{square root over (2)}W10{123}/{square root over (3)}W10{1324}/211u11 = [1 j −1 j]TW11{1}W11{13}/{square root over (2)}W11{134}/{square root over (3)}W11{1324}/212u12 = [1 −1 −1 1]TW12{1}W12{12}/{square root over (2)}W12{123}/{square root over (3)}W12{1234}/213u13 = [1 −1 1 −1]TW13{1}W13{13}/{square root over (2)}W13{123}/{square root over (3)}W13{1324}/214u14 = [1 1 −1 −1]TW14{1}W14{13}/{square root over (2)}W14{123}/{square root over (3)}W14{3214}/215u15 = [1 1 1 1]TW15{1}W15{12}/{square root over (2)}W15{123}/{square root over (3)}W15{1234}/2
The receiver measures a channel condition based on a reference signal received from the transmitter and selects a codebook based on phase information derived from the channel condition. The selected codebook is fed back to the transmitter in a codebook index indicative of the codebook. The codebooks specified in Table 1 are specified such that all the codebooks have uniform amplitude.
In Release 10, it is assumed that cross polarized antennas (CPAs) are used, and the receiver feeds two codebooks W1 and W2 representing the phase information back to the transmitter.
                    [                  Formula          ⁢                                          ⁢          1                ]                                                            W        =                                            W              1                        ⁢                          W              2                                =                                    [                                                                    X                                                        0                                                                                        0                                                        X                                                              ]                        ⁡                          [                                                                    Y                                                                                                                                      φ                        n                                            ⁢                      Y                                                                                  ]                                                          (        1        )            
Here, W1 represents a broadband/long-term channel quality, and W2 represents a subband/short-term channel quality.
As one example of the codebooks W1 and W2 in Formula (1), the following is presently proposed.
                    [                  Formula          ⁢                                          ⁢          2                ]                                                                                  X            =                          [                                                                    1                                                        1                                                                                                              ⅇ                                              2                        ⁢                        π                        ⁢                                                                                                  ⁢                        j                        ⁢                                                  m                          16                                                                                                                                                ⅇ                                              2                        ⁢                        π                        ⁢                                                                                                  ⁢                        j                        ⁢                                                                              m                            +                            1                                                    16                                                                                                                                ]                                ,                      m            =            0                    ,          1          ,          2          ,                      …            ⁢                                                  ⁢            15                          ⁢                                  ⁢                                            Y              ∈                                                {                                                            [                                                                                                    1                                                                                                                                0                                                                                              ]                                        ,                                          [                                                                                                    0                                                                                                                                1                                                                                              ]                                                        }                                ⁢                                                                  ⁢                                  φ                  n                                                      =                          ⅇ                              2                ⁢                π                ⁢                                                                  ⁢                j                ⁢                                  n                  8                                                              ,                      n            =            0                    ,          1          ,          …          ⁢                                          ,          7                                    (        2        )            
In the example in Formula (2), four bits (m=0, 1, . . . , 15) are used for feedback of W1, and four bits (1 bit for Y and three bits for n=0, 1, . . . , 7) are used for feedback of W2.
Also, as another example of the codebooks W1 and W2, the following is presently proposed.
                    [                  Formula          ⁢                                          ⁢          3                ]                                                                                  X            =                          [                                                                    1                                                                                                              ⅇ                                              2                        ⁢                        π                        ⁢                                                                                                  ⁢                        j                        ⁢                                                  m                          16                                                                                                                                ]                                ,                      m            =            0                    ,          1          ,          2          ,                      …            ⁢                                                  ⁢            15                          ⁢                                  ⁢                                            φ              n                        =                          ⅇ                              2                ⁢                π                ⁢                                                                  ⁢                j                ⁢                                  n                  4                                                              ,                      n            =            0                    ,          1          ,          2          ,          3                                    (        3        )            
In the example in Formula (3), four bits (m=0, 1, . . . , 15) are used for feedback of W1, and two bits (n=0, 1, 2, 3) are used for feedback of W2.
See 3GPP TS 36.211 V8.9.0 (2009-12) and 3GPP TS 36.211 V10.6.0 (2012-12) for further details, for example.